In general, the density of a print image may change with a change in ambient environment, such as temperature, or the progress of operating time in an image forming apparatus that prints an image on paper, such as a printer or a copying machine.
Accordingly, a known image forming apparatus, in many cases, forms a predetermined test pattern on an image carrier, such as a photoconductor or a transfer belt, and performs a density adjustment such that the density of the density test pattern falls within a predetermined reference range. For example, the density of the density test pattern formed on the image carrier is detected by a density sensor and adjustments of charged potential, development potential, exposure amount, and the like of the photoconductor are performed such that the detected density falls within a reference range. Usually, in such a density adjustment, a maximum gradation value or a value close to maximum gradation value is set as a set gradation value of the density test pattern in many cases. The density test pattern in this case is a so-called “solid pattern” or an equivalent pattern to the “solid pattern”. Therefore, the density adjustment is performed such that the maximum value of image data becomes a suitable print density.
The gradation characteristic from the maximum density to the minimum density also changes with the change in ambient environment, such as temperature, or the progress of operating time. The gradation is realized by selecting a plurality of gradation patterns with different densities according to the set gradation value, and the relationship between a level of denseness of a gradation pattern and the density obtained by printing is generally nonlinear. In order to correct this non-linearity, correction data called a gradation correction table is often used. The linearity of the characteristic (that is, gradation characteristic) between a gradation value of image data and the print density can be secured by interposing the gradation correction table between the gradation value of image data and a gradation value for selecting a gradation pattern.
However, the linearity of the gradation characteristic is also influenced by the intermediate density changing due to the temperature, operating time, and the like as described above. In order to maintain the linearity of the gradation characteristic, it is necessary to change and update the gradation correction table. An adjustment on the change and update is called a gradation adjustment separately from the density adjustment.
In a known gradation adjustment, first, a gradation test pattern gradually changing from low density to high density is printed on paper, next, the printed gradation test pattern is read with a scanner provided in an image forming apparatus, then, a gradation correction table is changed such that the density for every read gradation falls within a predetermined reference range. The gradation adjustment may be performed not only by a serviceman but also by a normal user. However, the work of printing a gradation test pattern on paper and reading the printed paper with the scanner is troublesome for a normal user. In practice, the work is rarely performed.
Moreover, in a known image forming apparatus, the density adjustment and the gradation adjustment are provided as separate functions, and the adjustments may be performed separately. However, a general gradation adjustment is performed by changing the shape of the gradation characteristic between the maximum gradation value and the minimum gradation value by changing the gradation correction table in order to maintain the linearity. Accordingly, when the maximum density corresponding to the maximum gradation value largely deviates from a proper value, it is not possible to set a desired gradation by change of the gradation correction table.